In a brazing process, a flux is an indispensable material which plays a very important role. The most crucial part in the flux is active components. An active agent in the flux is mainly used to remove an oxide film on the surface of a substrate to reduce a surface tension during soldering, increase wettability of the metal of solder and a soldering pad, thereby improving solderability.
The requirements for an ideal active agent are as follows: in general cases, the active agent is stable, and does not react with a substrate even being in contact with the substrate; during the course of using, when heated up to a certain temperature, the active material starts to melt and wet the solder and the soldering pad; with a further increase of the temperature, the active material starts to gradually release its activating capability; when reaching the optimum soldering temperature, all the activating capability is released; after completion of soldering, with a decrease of the temperature, the remaining active material returns to an original inert state; in the course of soldering, salts such as copper salt, tin salt and other salts, generated through reacting with an oxide film, should not influence electrical performance; of course, should not corrode the substrate.
Generally, a current active agent may be an inorganic acid salt of organic substances, or an organic acid, or mixtures thereof. Most commonly used inorganic acid salts are halogen acid salts, such as amine hydrochlorides, guanidine hydrochlorides, amine hydrobromides, guanidine hydrobromides, etc. All of them have a characteristic of high activity. However, their defects are fatal: corroding the substrate and severely influencing the electrical performance. The active agent of organic acids is currently a development hotspot, with continuous appearance of patents, such as those disclosed in U.S. Pat. No. 5,507,882, 1996 (“Bristol Samuel V Low residue water-based soldering flux and process for soldering with same”), Pat. CN1209374A, 1999 (Shengbo LI, “No-clean flux with low solid content”), Pat. CN1398698A, 2003 (Hesheng DENG et al, “No-clean liquid flux”), Pat. CN1011564805A, 2009 (Yongping LEI et al, “Novel environment-friendly flux for SnAgCu lead-free soldering paste with low Ag content). Generally, an organic carboxylic acid, especially dicarboxylic acid such as butane diacid, pentane diacid and hexane diacid, is preferably used to solve the problem of corrosion in using halogen active agents and the problem of electrical performance generated therefrom. Indeed, the problem is solved. Unfortunately, the activity is greatly reduced, which is not comparable with the activity of halogen active agents at all. The compromised solution is as follows: combining the organic acid and halogen and finding a balance between the corrosion and activity, such as those disclosed in Pat. CN101543943A, 2009 (Junhu QIN et al, “No-clean lead-free rosin-core flux with low halogen content and preparation method thereof”), Pat. CN101569966A, 2009 (Jian WU et al, “A lead-free solder paste and method for preparing its flux”) etc. The solution can only be applied to a variety with low performance requirements, but not to a variety with high performance requirements.